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I want to move to Sprite Kit framework offered by apple which has physics integrated right into it. The back bone of the physics engine is famous Box2D. Sprite Kit has made it pretty easy by wrapping C++ inside Objective-C but unfortunately some features lack in Sprite Kit. One them is GetReactionForce function, that returns the impulse applied on the body B of a joint. Just disregard the software related terms if you find it difficult to understand. Basically what I need is to create my own function that will calculate impulse. Here are the ingredients I have: Mass, gravity, delta time.

P.S. Sorry for the tags, I couldn't find anything more suitable and I have no right to create new ones

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closed as off-topic by DumpsterDoofus, jinawee, Brandon Enright, Kyle Kanos, Waffle's Crazy Peanut Mar 16 at 14:03

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Hi, and welcome to StackExchange Physics! If I understand what you're asking correctly, this is something which is typically taught in basic high-school freshman physics, so you could start by reviewing your high-school notes. Also, we expect you to attempt the problem on your own, elucidate which issues you do and don't understand, search through the question archives (using the search bar) for relevant previous questions, and try to highlight which broad conceptual issue you are having before asking a question here. –  DumpsterDoofus Mar 15 at 13:53
    
Brief answer to question: in general, the impulse (or change in linear momentum imparted to a body of mass $m$) under the influence of gravity acceleration $g$ for a duration $\Delta t$ is $\Delta p=mg\Delta t$. Is that what you're looking for, or are you trying to calculate something different? –  DumpsterDoofus Mar 15 at 13:54
    
Dear, @DumpsterDoofus, my physics at high school were never good. :) That's why I'm asking a question that sounds stupid to you. In fact, I know the formula. I just asked it here because I thought I might miss something other than what a bare formula could give me. –  Mike JM Mar 15 at 14:28

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up vote 1 down vote accepted

I hope I understand the question: you are looking for the impulse on an object of mass $m$ due to the force of gravity while it experiences the force for a time interval $\Delta t$.

Let's assume that gravity points in the $-\hat{z}$ direction. Impulse is a vector quantity. Since the force points "down", impulse will have no components in the $\hat{x}$ and $\hat{y}$ directions: $I_x = I_y = 0$.

For a constant force like gravity, $I_z = F_z\Delta t$, so $I_z = -mg\Delta t$. Hope that helps.

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Thank you @garyp. I think this is what I need. –  Mike JM Mar 15 at 14:31

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